flattenAlongPlane.py

import pymel.core as pymel
import maya.api.OpenMaya as om

def flattenAlongPlane(joints):
  # Get worldspace position every joint but for first and last, and get their average position
  vectors = [om.MVector(pymel.joint(x,q=True, p=True, a=True)) for x in joints[1:-1]]    
  avg = om.MPoint([sum(v)/len(vectors) for v in zip(*vectors)])
  p1 = om.MPoint(pymel.joint(joints[0], q=True, p=True, a=True))
  p2 = om.MPoint(pymel.joint(joints[-1], q=True, p=True, a=True))

  # Get normal, by getting cross product of avg point to both start and end point
  normal = om.MVector(avg - p1) ^ om.MVector(avg - p2)

  # Create a plane and set it's normal
  mPlane = om.MPlane()
  mPlane.setPlane(normal.normalize(), 0)

  # Get distance to our avg point
  d = mPlane.distanceToPoint(om.MVector(avg))

  # Set plane to our point average
  mPlane.setPlane(normal.normalize(), d)

  # Unparent last joint to not have translation propage further than selected section
  pymel.parent(joints[-1], world = True)

  for joint in joints[1:-1]:
    pos = om.MVector(pymel.joint(joint, q=True, p=True, a=True))
    distance = mPlane.distanceToPoint(pos)

    # Dot product to check if joint is below or above desired plane.
    r = normal * om.MVector(om.MPoint(pos) - avg)

    nPos = None

    if r > 0:
      nPos = om.MVector(pos - om.MVector(normal*distance))
    elif r < 0:
      nPos = om.MVector(pos - om.MVector(normal*-distance))
    else:
      # Point is on our plane
      pass

    # Unsure about me flagging for children, don't think I am using 
    pymel.joint(joint, e = True, p = nPos, a = True)

  pymel.parent(joints[-1], joints[-2])